Heterogeneous reactions, i.e. the reactions of a fluid phase (e.g. a gas) on a solid surface are commonly catalyzed by a contact catalyst mass of selected composition.
In practice, the effectiveness of a contact catalyst for heterogeneous reactions depends not only upon the composition of the mass but also upon the nature and composition of its support and hence the combination of the two. In other words the poor selection of a support will result in diminution or elimination of the activity of the catalyzer in certain zones of the catalyst. Similarly a support which is effective for certain catalytically active materials will be detrimental to others or will not allow reaction upon these other materials.
The specificity of the catalytically active material with respect to its support increases as the sensitivity to the reaction conditions of this material becomes more pronounced. For example, the choice of a support becomes more important as the reaction becomes more sensitive because of a reduced activity or affinity of the reactants for one another under certain temperature, pressure or reactant-concentration conditions, or the like.
In catalytic combustion, compounds such as ethane, butane, propane and heptane are effectively oxidized at relatively wide-ranging temperatures and high concentrations, while methane may only be oxidized with difficulty. This can be attributed in large measure to the low affinity of methane and oxygen in the presence of a catalyst, especially at very low temperatures.
The oxidations reaction can be improved by using catalyst supports having an especially large specific surface and specially designed structure or texture, e.g. alumina wool. A catalyst mass on such a support is capable of effecting a conversion of methane close to unity (100%) even under moderate temperature conditions. The mass is thus compatible with use in catalytic heaters or other apparatus designed for flameless combustion at low temperatures.
However the alumina fibers and the contact mass employed therewith create certain problems which can only be overcome with considerable difficulty. These include the migration of elements of the catalyst mass on the fibers and the retention of various salts of active elements of the catalyzer in solution on the fibers.
These physical phenomena reduce in large measure the efficiency of the active mass unless they are controlled rigorously and can require the use of larger catalyst masses than would otherwise be the case. This adds significantly to the operating and purchase cost of the catalytic materials.
It is possible to use as supports other refractory materials such as asbestos, silica wool and metal-wire cloth which are less effective in the catalytical oxidation of methane.
Insufficient activity of the catalytic mass generally results in a high output of non-oxidized gas and hence the carbon dioxide produced is accompanied by compounds of lesser degrees of oxidation such as aldehydes and acids. Such reduced activity is essentially a result of low or lowered specific surface of the material constituting the support or of a modification of the surface texture so that the contact surface presented to the reactants is reduced.
It has also been proposed to use as supports materials of a highly advantageous texture such as silico-aluminous (silica-alumina) fibers in which the silica and alumina components are present in substantially equal proportions. While such supports give excellent results for the catalytic oxidation of propane and butane, for example, they are not always satisfactory for the catalytic oxidation of methane. The lesser activity as to methane can be partially compensated in increasing significantly the proportions of the catalyzer in the catalyst mass or by substituting for the catalyzers usually used, catalyzers consisting only of noble metals such as platinum. Because of their cost, however, such catalyst masses are prohibitive for industrial purposes.
In this connection reference may be had to my earlier U.S. Pat. No. 4,029,602 of June 14, 1977 and No. 4,048,113 of Sept. 13, 1977 and the art, publications and references of record therein.